A known trocar in the related art is formed of two tube members that are connected so as to be able to swivel relative to each other (for example, see Patent Literature 1).
This trocar provides a wide working space even when a portion to be treated is located at a shallow position from a body surface, by placing the trocar through the body surface tissue of a patient such that a distal-end tube member is disposed inside the body and a proximal-end tube member is disposed outside the body, and by swiveling the distal-end tube member relative to the proximal-end tube member while a medical device, such as an endoscope, is inserted through the inner hole of the trocar.